Strand-like materials can be metal wire for example, that may be used as a conductor of a cable. Typically a plastic insulation is extruded onto such wire in an extrusion device. For a proper application and adhesion of the extrudate on the wire, it is necessary for the wire to have a predetermined temperature. Therefore, the wires are generally heated to a target temperature in a preheating unit before the extrusion process. Such a preheating unit is sold by Sikora AG, for example, under the name PREHEATER 6000. With this unit, the wire is guided about two disks that are mounted so as to rotate, of which at least one is electrically conducting, and thus causes a short circuit in the wire loop guided about the disks. The wire can then be heated to the target temperature by means of a heating apparatus disposed between the disks.
The design of a plastic insulation applied in the scope of an extrusion method depends sensitively on the temperature of the wire. Particularly with foaming material applied in the scope of an extrusion, fluctuations in the temperature of the wire can lead to non-uniform thickness of the plastic insulation. Fluctuations in the plastic insulation especially with data cables can lead to capacitance changes, and with it, to changes of the characteristic wave impedance. This in turn, can lead to undesired reflection loss. Furthermore, deviations from the target temperature can also lead to problems with respect to the adhesion of the plastic material to the wire.
It has already been proposed to measure the temperature of the wire after passing through a preheating apparatus. Known measurement methods are based on the evaluation of the temperature-dependent emission behavior of the wire. Specifically with copper wire, which is frequently to be measured in this context, the emission behavior can vary significantly. This results in inaccuracies with the temperature measurement and thus in problems with a subsequent extrusion process.